My grandmother was a widow on the 1921 Scotland Census. The letters F.D. are in the Marriage/Orphanhood Column for my mother and uncles. I assume this means Farther Deceased. I've also noticed M.D. (Mother Deceased) and Illig. on some pages. Adopted children have 'Adopted Son' or 'Adopted Daughter' in the Relationship Column.
Regards, Dod.